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420 lines
15 KiB
420 lines
15 KiB
#lang ivy1.7
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# ---
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# layout: page
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# title: Specification of Tendermint in Ivy
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# ---
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# This specification closely follows the pseudo-code given in "The latest
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# gossip on BFT consensus" by E. Buchman, J. Kwon, Z. Milosevic
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# <https://arxiv.org/abs/1807.04938>
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include domain_model
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include network_shim
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# We model the Tendermint protocol as an Ivy object. Like in Object-Oriented
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# Programming, the basic structuring unit in Ivy is the object. Objects have
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# internal state and actions (i.e. methods in OO parlance) that modify their
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# state. We model Tendermint as an object whose actions represent steps taken
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# by individual nodes in the protocol. Actions in Ivy can have preconditions,
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# and a valid execution is a sequence of actions whose preconditions are all
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# satisfied in the state in which they are called.
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# For technical reasons, we define below a `tendermint` module instead of an
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# object. Ivy modules are a little bit like classes in OO programs, and like
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# classes they can be instantiated to obtain objects. To instantiate the
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# `tendermint` module, we must provide an abstract-protocol object. This allows
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# us to use different abstract-protocol objects for different parts of the
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# proof, and to do so without too much notational burden (we could have used
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# Ivy monitors, but then we would need to prefix every variable name by the
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# name of the object containing it, which clutters things a bit compared to the
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# approach we took).
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# The abstract-protocol object is called by the resulting tendermint object so
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# as to run the abstract protocol alongside the concrete protocol. This allows
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# us to transfer properties proved of the abstract protocol to the concrete
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# protocol, as follows. First, we prove that running the abstract protocol in
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# this way results in a valid execution of the abstract protocol. This is done
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# by checking that all preconditions of the abstract actions are satisfied at
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# their call sites. Second, we establish a relation between abstract state and
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# concrete state (in the form of invariants of the resulting, two-object
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# transition system) that allow us to transfer properties proved in the
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# abstract protocol to the concrete protocol (for example, we prove that any
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# decision made in the Tendermint protocol is also made in the abstract
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# protocol; if the abstract protocol satisfies the agreement property, this
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# allows us to conclude that the Tendermint protocol also does).
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# The abstract protocol object that we will use is always the same, and only
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# the abstract properties that we prove about it change in the different
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# instantiations of the `tendermint` module. Thus we provide common invariants
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# that a) allow to prove that the abstract preconditions are met, and b)
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# provide a refinement relation (see end of the module) relating the state of
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# Tendermint to the state of the abstract protocol.
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# In the model, Byzantine nodes can send whatever messages they want, except
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# that they cannot forge sender identities. This reflects the fact that, in
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# practice, nodes use public key cryptography to sign their messages.
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# Finally, note that the observations that serve to adjudicate misbehavior are
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# defined only in the abstract protocol (they happen in the abstract actions).
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module tendermint(abstract_protocol) = {
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# the initial value of a node:
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function init_val(N:node): value
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# the three type of steps
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object step_t = {
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type this = {propose, prevote, precommit}
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} # refer to those e.g. as step_t.propose
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object server(n:node) = {
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# the current round of a node
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individual round_p: round
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individual step: step_t
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individual decision: value
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individual lockedValue: value
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individual lockedRound: round
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individual validValue: value
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individual validRound: round
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relation done_l34(R:round)
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relation done_l36(R:round, V:value)
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relation done_l47(R:round)
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# variables for scheduling request
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relation propose_timer_scheduled(R:round)
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relation prevote_timer_scheduled(R:round)
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relation precommit_timer_scheduled(R:round)
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relation _recved_proposal(Sender:node, R:round, V:value, VR:round)
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relation _recved_prevote(Sender:node, R:round, V:value)
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relation _recved_precommit(Sender:node, R:round, V:value)
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relation _has_started
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after init {
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round_p := 0;
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step := step_t.propose;
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decision := value.nil;
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lockedValue := value.nil;
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lockedRound := round.minus_one;
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validValue := value.nil;
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validRound := round.minus_one;
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done_l34(R) := false;
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done_l36(R, V) := false;
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done_l47(R) := false;
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propose_timer_scheduled(R) := false;
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prevote_timer_scheduled(R) := false;
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precommit_timer_scheduled(R) := false;
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_recved_proposal(Sender, R, V, VR) := false;
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_recved_prevote(Sender, R, V) := false;
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_recved_precommit(Sender, R, V) := false;
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_has_started := false;
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}
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action getValue returns (v:value) = {
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v := init_val(n)
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}
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export action start = {
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require ~_has_started;
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_has_started := true;
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# line 10
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call startRound(0);
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}
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# line 11-21
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action startRound(r:round) = {
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# line 12
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round_p := r;
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# line 13
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step := step_t.propose;
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var proposal : value;
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# line 14
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if (proposers.get_proposer(r) = n) {
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if validValue ~= value.nil { # line 15
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proposal := validValue; # line 16
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} else {
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proposal := getValue(); # line 18
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};
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call broadcast_proposal(r, proposal, validRound); # line 19
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} else {
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propose_timer_scheduled(r) := true; # line 21
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};
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call abstract_protocol.l_11(n, r);
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}
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# This action, as not exported, can only be called at specific call sites.
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action broadcast_proposal(r:round, v:value, vr:round) = {
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var m: msg;
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m.m_kind := msg_kind.proposal;
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m.m_src := n;
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m.m_round := r;
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m.m_value := v;
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m.m_vround := vr;
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call shim.broadcast(n,m);
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}
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implement shim.proposal_handler.handle(msg:msg) {
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_recved_proposal(msg.m_src, msg.m_round, msg.m_value, msg.m_vround) := true;
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}
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# line 22-27
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export action l_22(v:value) = {
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require _has_started;
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require _recved_proposal(proposers.get_proposer(round_p), round_p, v, round.minus_one);
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require step = step_t.propose;
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if (value.valid(v) & (lockedRound = round.minus_one | lockedValue = v)) {
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call broadcast_prevote(round_p, v); # line 24
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call abstract_protocol.l_22(n, round_p, v);
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} else {
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call broadcast_prevote(round_p, value.nil); # line 26
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call abstract_protocol.l_22(n, round_p, value.nil);
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};
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# line 27
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step := step_t.prevote;
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}
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# line 28-33
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export action l_28(r:round, v:value, vr:round, q:nset) = {
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require _has_started;
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require r = round_p;
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require _recved_proposal(proposers.get_proposer(r), r, v, vr);
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require nset.is_quorum(q);
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require nset.member(N,q) -> _recved_prevote(N,vr,v);
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require step = step_t.propose;
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require vr >= 0 & vr < r;
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# line 29
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if (value.valid(v) & (lockedRound <= vr | lockedValue = v)) {
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call broadcast_prevote(r, v);
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} else {
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call broadcast_prevote(r, value.nil);
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};
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call abstract_protocol.l_28(n,r,v,vr,q);
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step := step_t.prevote;
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}
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action broadcast_prevote(r:round, v:value) = {
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var m: msg;
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m.m_kind := msg_kind.prevote;
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m.m_src := n;
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m.m_round := r;
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m.m_value := v;
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call shim.broadcast(n,m);
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}
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implement shim.prevote_handler.handle(msg:msg) {
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_recved_prevote(msg.m_src, msg.m_round, msg.m_value) := true;
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}
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# line 34-35
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export action l_34(r:round, q:nset) = {
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require _has_started;
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require round_p = r;
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require nset.is_quorum(q);
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require exists V . nset.member(N,q) -> _recved_prevote(N,r,V);
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require step = step_t.prevote;
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require ~done_l34(r);
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done_l34(r) := true;
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prevote_timer_scheduled(r) := true;
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}
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# line 36-43
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export action l_36(r:round, v:value, q:nset) = {
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require _has_started;
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require r = round_p;
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require exists VR . round.minus_one <= VR & VR < r & _recved_proposal(proposers.get_proposer(r), r, v, VR);
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require nset.is_quorum(q);
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require nset.member(N,q) -> _recved_prevote(N,r,v);
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require value.valid(v);
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require step = step_t.prevote | step = step_t.precommit;
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require ~done_l36(r,v);
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done_l36(r, v) := true;
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if step = step_t.prevote {
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lockedValue := v; # line 38
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lockedRound := r; # line 39
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call broadcast_precommit(r, v); # line 40
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step := step_t.precommit; # line 41
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call abstract_protocol.l_36(n, r, v, q);
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};
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validValue := v; # line 42
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validRound := r; # line 43
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}
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# line 44-46
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export action l_44(r:round, q:nset) = {
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require _has_started;
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require r = round_p;
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require nset.is_quorum(q);
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require nset.member(N,q) -> _recved_prevote(N,r,value.nil);
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require step = step_t.prevote;
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call broadcast_precommit(r, value.nil); # line 45
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step := step_t.precommit; # line 46
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call abstract_protocol.l_44(n, r, q);
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}
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action broadcast_precommit(r:round, v:value) = {
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var m: msg;
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m.m_kind := msg_kind.precommit;
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m.m_src := n;
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m.m_round := r;
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m.m_value := v;
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call shim.broadcast(n,m);
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}
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implement shim.precommit_handler.handle(msg:msg) {
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_recved_precommit(msg.m_src, msg.m_round, msg.m_value) := true;
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}
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# line 47-48
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export action l_47(r:round, q:nset) = {
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require _has_started;
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require round_p = r;
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require nset.is_quorum(q);
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require nset.member(N,q) -> exists V . _recved_precommit(N,r,V);
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require ~done_l47(r);
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done_l47(r) := true;
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precommit_timer_scheduled(r) := true;
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}
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# line 49-54
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export action l_49_decide(r:round, v:value, q:nset) = {
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require _has_started;
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require exists VR . round.minus_one <= VR & VR < r & _recved_proposal(proposers.get_proposer(r), r, v, VR);
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require nset.is_quorum(q);
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require nset.member(N,q) -> _recved_precommit(N,r,v);
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require decision = value.nil;
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if value.valid(v) {
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decision := v;
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# MORE for next height
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call abstract_protocol.decide(n, r, v, q);
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}
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}
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# line 55-56
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export action l_55(r:round, b:nset) = {
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require _has_started;
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require nset.is_blocking(b);
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require nset.member(N,b) -> exists VR . round.minus_one <= VR & VR < r & exists V . _recved_proposal(N,r,V,VR) | _recved_prevote(N,r,V) | _recved_precommit(N,r,V);
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require r > round_p;
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call startRound(r); # line 56
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}
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# line 57-60
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export action onTimeoutPropose(r:round) = {
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require _has_started;
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require propose_timer_scheduled(r);
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require r = round_p;
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require step = step_t.propose;
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call broadcast_prevote(r,value.nil);
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step := step_t.prevote;
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call abstract_protocol.l_57(n,r);
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propose_timer_scheduled(r) := false;
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}
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# line 61-64
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export action onTimeoutPrevote(r:round) = {
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require _has_started;
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require prevote_timer_scheduled(r);
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require r = round_p;
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require step = step_t.prevote;
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call broadcast_precommit(r,value.nil);
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step := step_t.precommit;
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call abstract_protocol.l_61(n,r);
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prevote_timer_scheduled(r) := false;
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}
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# line 65-67
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export action onTimeoutPrecommit(r:round) = {
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require _has_started;
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require precommit_timer_scheduled(r);
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require r = round_p;
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call startRound(round.incr(r));
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precommit_timer_scheduled(r) := false;
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}
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# The Byzantine actions
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# ---------------------
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# Byzantine nodes can send whatever they want, but they cannot send
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# messages on behalf of well-behaved nodes. In practice this is implemented
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# using cryptography (e.g. public-key cryptography).
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export action byzantine_send(m:msg, dst:node) = {
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require ~well_behaved(n);
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require ~well_behaved(m.m_src); # cannot forge the identity of well-behaved nodes
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call shim.send(n,dst,m);
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}
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# Byzantine nodes can also report fake observations, as defined in the abstract protocol.
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export action fake_observations = {
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call abstract_protocol.misbehave
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}
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# Invariants
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# ----------
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# We provide common invariants that a) allow to prove that the abstract
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# preconditions are met, and b) provide a refinement relation.
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specification {
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invariant 0 <= round_p
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invariant abstract_protocol.left_round(n,R) <-> R < round_p
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invariant lockedRound ~= round.minus_one -> forall R,V . abstract_protocol.locked(n,R,V) <-> R <= lockedRound & lockedValue = V
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invariant lockedRound = round.minus_one -> forall R,V . ~abstract_protocol.locked(n,R,V)
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invariant forall M:msg . well_behaved(M.m_src) & M.m_kind = msg_kind.prevote & shim.sent(M,N) -> abstract_protocol.prevoted(M.m_src,M.m_round,M.m_value)
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invariant well_behaved(N) & _recved_prevote(N,R,V) -> abstract_protocol.prevoted(N,R,V)
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invariant forall M:msg . well_behaved(M.m_src) & M.m_kind = msg_kind.precommit & shim.sent(M,N) -> abstract_protocol.precommitted(M.m_src,M.m_round,M.m_value)
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invariant well_behaved(N) & _recved_precommit(N,R,V) -> abstract_protocol.precommitted(N,R,V)
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invariant (step = step_t.prevote | step = step_t.propose) -> ~abstract_protocol.precommitted(n,round_p,V)
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invariant step = step_t.propose -> ~abstract_protocol.prevoted(n,round_p,V)
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invariant step = step_t.prevote -> exists V . abstract_protocol.prevoted(n,round_p,V)
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invariant round_p < R -> ~(abstract_protocol.prevoted(n,R,V) | abstract_protocol.precommitted(n,R,V))
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invariant ~_has_started -> step = step_t.propose & ~(abstract_protocol.prevoted(n,R,V) | abstract_protocol.precommitted(n,R,V)) & round_p = 0
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invariant decision ~= value.nil -> exists R . abstract_protocol.decided(n,R,decision)
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}
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}
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}
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